Upconversion lasers provide a promising alternative to frequency-doubled laser diodes for a reliable and efficient compact source of blue or green laser light. Unlike the conventional single-step optical pump process, in which absorption of one pump photon is sufficient to excite an active ion to the upper laser level and in which the laser light always has a lower photon energy, i.e. a longer wavelength, than the pump light, the upconversion pump process is a multi-step process in which more than one photon excites an active ion to the upper laser level and in which the laser light usually has a shorter wavelength than the pump light. Common upconversion processes include excited-state absorption of a second photon by the active ion, also called "two-step absorption", dipole-dipole cross-relaxation interactions between two excited ions, not necessarily of the same atomic species, and avalanche absorption involving a combination of both excited-state absorption and interionic cross-relaxation.
Upconversion solid-state lasers using rare-earth-doped crystal or glass rods as the active medium have been demonstrated. In U.S. Pat. No. 3,624,549, Geusic describes diode pumped solid state upconversion lasers in which a crystal rod is doped with Er.sup.3+, Ho.sup.3+ or Tm.sup.3+ active ions and co-doped with Yb.sup.3+ sensitizer ions. The sensitizer ions absorb the 0.93 .mu.m emission from silicon and indium doped GaAs diode pumps, and transfer the energy to the lasing active ions to produce 550 nm, 660 nm or 410 nm laser emission for erbium (Er.sup.3+), 540 nm laser emission for holmium (Ho.sup.3+), and 470 nm laser emission for thullium (Tm.sup.3+), respectively. In U.S. Pat. No. 4,949,348, Nguyen et al. describe a thullium-doped solid-state upconversion laser which is double-pumped with both 781 nm GaAlAs and 649 nm InGaAlP laser diodes to produce a 450 nm laser emission. In U.S. Pat. No. 5,008,890, McFarlane describes an erbium-doped solid-state upconversion laser which is pumped either by a 797 nm laser diode or by a diode-laser-pumped Nd.sup.3+ -doped 1.06 .mu.m solid state laser. The erbium upconversion laser lases at 551 nm or one of several other wavelengths.
Upconversion fiber lasers, using rare-earth-doped single-mode fluoride-glass fibers, have also been demonstrated. In U.S. Pat. No. 5,067,134, Oomen describes a diode-laser-pumped thullium upconversion fiber laser which lases at about 450 nm wavelength. The fiber can be co-doped with terbium or praseodymium. In U.S. Pat. No. 5,226,049, Grubb describes thullium and holmium upconversion fiber lasers. The pump source may be either a laser diode emitting 1120 nm radiation or a Nd:YAG laser which has been adjusted to suppress 1064 nm and 1074 nm emissions and to enhance 1112 nm, 1116 nm and 1123 nm output. The thullium laser produces 480 nm or 650 nm laser light, while the holmium laser produces 550 nm laser light. T. J. Whitley et al., in Electronics Letters 27 (20), 26th Sep. 1991, pages 1785-1786, describe an erbium upconversion fiber laser pumped by an 801 nm laser diode and emitting 546 nm laser radiation. D. Piehler et al., in an article entitled "Green laser diode pumped erbium fiber laser" presented at the Compact Blue-Green Lasers conference, held Feb. 10-11, 1994 at Salt Lake City, Utah, describes work done in 1993 on an erbium-doped fluoride fiber upconversion laser which is pumped with a 971 nm laser diode and which emits 544 nm laser radiation. The laser diode pump source is fiber coupled to the upconversion fiber. Piehler et al. also note the prior art's simultaneous pumping of a praseodymium-doped fluoride fiber upconversion laser with both an 833 nm laser diode and either a 985 nm or 1016 nm laser diode to produce either 635 nm or 521 nm laser emission. The fiber core in that laser is co-doped with ytterbium.
W. Lenth et al. in Optics and Photonics News, March 1992, pages 8-15, survey prior work on solid state and fiber upconversion lasers. Among the systems noted in the article are thullium lasers pumped with 648 nm light or double pumped with both 647 nm and 676 nm light and lasing at 480 nm or 483 nm. Erbium lasers are pumped with 800 nm light or double pumped with both 792 nm and 840 nm light and lase at 550 nm. Praseodymium lasers are double pumped with both 835 nm and 1010 nm light and lase at 491 nm, 520 nm, 605 nm, 635 nm or 715 nm wavelengths.
D. Piehler, in Laser Focus World, November 1992, pages 95-102, also surveys prior art upconversion fiber lasers. Among the lasers disclosed are a 480 nm thullium laser pumped with a diode-pumped Nd:YAG fiber laser providing 1120 nm pump radiation. Also, a praseodymium laser, lasing at 635 nm, 520 nm or 490 nm, is pumped by a diode-laser-pumped ytterbium fiber laser. The ytterbium fiber laser pump employs an intrafiber Bragg reflector to force emission of 1020 nm pump radiation.
Diode-laser-pumped single-mode rare-earth-doped fluoride-glass fibers are the preferred medium for upconversion. Upconversion efficiency increases with pump intensity. Single-mode fibers (10 .mu.m core diameter or less) can confine the pump and laser radiation to a very small area over the length of these fibers (often several meters long), and create very high optical intensities and large single-pass gains from only modest pump powers. This makes room temperature cw operation of an upconversion laser possible. In comparison with solid-state crystal media, the glass medium used for fiber lasers has broader absorption and emission bands, so the laser diode pump sources need not be frequency stabilized and the laser output is tunable to some degree (over approximately a 10 nm range). Fluoride glass fibers are especially advantageous, because the lower phonon energies, and thus smaller vibrational decay rates, of this material lead to increased excited-state lifetimes and a relative abundance of the metastable intermediate states required for effective upconversion.
An object of the invention is to increase the amount of usable pump light injected into an upconversion laser fiber for improved upconversion efficiency and higher laser power output.
Another object of the invention is to provide an upconversion fiber laser which can employ higher brightness pump sources than previously possible.